1. Field of the Invention
The present invention relates to a printer, in particular, a printer that detects lateral edges of a recording paper to realize margin-free printing.
2. Explanations of the Prior Arts
Due to the wide usage of digital still camera, demand for color printing of the photographed image is on the increase. A color thermosensitive printer is used for color printing. The color thermosensitive printer presses a heating element array of a thermal head to a color thermosensitive recording paper with yellow, magenta, and cyan thermosensitive coloring layers on a substrate, so that the three thermosensitive coloring layers successively develop their respective color, to form a full-color image. It is preferable to have a marginless printing to make the most of a recording area, as well as making the appearance of the color printing better. In this case, the heating element array must be wider than the color thermosensitive recording paper to record the lateral edge of the color thermosensitive recording paper without fail. Heating the heating element that does not touch the color thermal recording paper causes xe2x80x9cwasted heatingxe2x80x9d, which shortens the life of the heating element. In order to prevent this, JPA No. 9-272217 discloses a printer that applies inspection light to a lateral edge portion of a color thermosensitive recording paper. A CCD line sensor detects inspection light reflected at the color thermosensitive recording paper, so that the lateral edge is detected. The detection error range is increased when the illuminance of the light source is high beyond a range of linearity of the CCD line sensor, or low enough to be affected by a dark electrical current and a noise.
In order to detect the edge position of the color thermal recording paper precisely, it is necessary to set the illuminance of the light source within a range of linearity of the CCD line sensor. It is also necessary to keep the illuminance of the light source at a regular level as an output level from the CCD line sensor is changed in accordance with the intensity of illumination. Moreover, an electrical charge storage period must be kept constant because an output from the CCD line sensor is increased or decreased when the electrical charge storage period is long or short. However, fluctuation in illuminance in the light source make it difficult to keep the output level of the CCD line sensor within the linearlity.
The printing period for each line is different in accordance with the sensitivity of the thermosensitive coloring layers. Due to the difference in the printing period, the electrical charge storage period is changed in printing to each thermosensitive coloring layer. Furthermore, reflectance of the thermosensitive recording paper changes in accordance of the type, such as a normal type and a seal type. Thus, the output level of the CCD line sensor is changed, which makes it difficult to detect lateral edges of the color thermal recording paper precisely.
An object of the present invention is to detect the lateral edge of a recording paper precisely by keeping an output of a sensor at a certain level within its linearlity.
To attain the above object, the printer of the present invention is provided with a light control circuit and a control means.
The light control circuit controls illumination of inspection light irradiated from the light source based on the output signal level from the sensor. And the control means detects a lateral edge of a recording paper on receiving a signal from the CCD line sensor, and records an image by driving plural recording elements in contact with the recording paper in a main scanning direction.
In the preferred embodiment, the sensor is a CCD line sensor in which plural pixels are arranged along the main scanning direction. The control means detects the lateral edge of the recording paper by comparing the output signal level from the pixels. The light control circuit conducts illumination adjustment of inspection light prior to recording the image. The light control circuit fixes a target output value of the output signal level to change illumination of inspection light so that the output signal level from the pixel becomes close to the target output value. At this time, the light control circuit detects the signal from each pixel by a one line recording period for recording the image by one line.
As the output signal level from the CCD line sensor is kept constant in this way, the lateral edge of the color thermosensitive paper can be detected precisely.